The thermophobic character is a temperature mutant which is unable to grow at35°C on certain sugars in tube culture. This character is controlled by a singlegene located far away from the centromere on the chromosome of Paba linkage group.
A limited growth of thermophobic strain on lactose minimal medium can beforced at 35° after activation at 25° for as little as 2 hours. The experimentsshow that the inhibition of growth at higher temperature is not due to nutritionaldeficiency, change of permeability of cell membrane to certain sugars or inhibitionof hydrolytic enzymes of oligosaccharides. At 40° the thermophobic strain cannot grow on any sugar, while the normal strain can grow on certain sugars but notothers, just as the thermophobic strain did at 35°. The difference of effectivetemperature between the strains is only 5°C. This led to the studies of thecommon step of carbohydrate degradation, e.g., glycolysis and Krebs' cycle. AllKrebs' acids were found to have a strong thermophobic effect at 35° on the thermophobicstrain on sucrose. But the manometric studies of certain of the enzymesystems in Krebs' cycle reveal no significant differences between the strains.
In liquid culture on lactose at 35° there appears a morphological differencebetween the strains, i.e., semicolonial for the thermophobic strain and usualfilamentous for the normal. This led to the studies of cell wall material inNeurospora, and it was found that there is a difference in response to chlorozinciodine staining. The staining reaction is quantitatively related to the length ofunacetylated end group of chitin. Estimates show that the chitin of the thermophobicstrain has a length of half molecule only about 1/5 that of normal. A length ofchitin half mol ecule less than 100 acetylglucosamine units will induce a growthof semicolonial form in liquid culture, and will have very little measurable growthin tube culture. The length of chitin molecule is influenced by glucose-acetateratio. Temperature and type of sugar will influence the ratio greatly, and willthus change the habit of growth of both strains, but different quantitatively asthe thermophobic gene causes a shortness of chitin molecule from the start.
